1. Field of the Invention
The present invention relates to an optical receiver for an optical transmission system, and more particularly to an apparatus for controlling a decision threshold voltage to an optical receiver, which is capable of automatically controlling the decision threshold voltage to the optical receiver appropriately to signal level decision on the basis of a low-frequency band signal component of an output signal from the optical receiver so as to reduce a bit error rate, and being implemented in a simple manner at low cost.
2. Description of the Related Art
In general, an optical transmission system comprises an optical transmitter module for converting an electrical signal into an optical signal, an optical fiber cable for transmitting the optical signal from the optical transmitter module, a repeater for amplifying and transmitting the optical signal from the optical fiber cable in the middle of its transmission by the cable, and an optical receiver module for converting the optical signal amplified and transmitted by the repeater back to an electrical signal. The optical receiver module includes a clock/data recovery unit, which will hereinafter be described with reference to FIG. 1.
FIG. 1 is a block diagram showing the configuration of a conventional optical receiver module.
With reference to FIG. 1, the conventional optical receiver module includes an optical receiver 110 for converting an input optical signal Sin into an electrical signal, a clock/data recovery unit 120 for receiving an output signal Scout from the optical receiver 110 and recovering a clock and data contained in the received signal Scout, a bit error rate tester 130 for measuring a bit error rate of the data using a data signal SD and clock signal SC from the clock/data recovery unit 120, and a digital oscilloscope 140 for measuring a cross point of the output signal Scout from the optical receiver 110.
In order to measure optical transmission performance of the optical transmission system, there is a need to perform a photoelectric conversion function and a clock/data recovery function, respectively, using the optical receiver and the clock/data recovery unit. An optimum condition for level decision in the clock/data recovery unit, or an optimum 1/0 distribution condition, can be realized by optimally adjusting distributions of 1 and 0 levels of the electrical signal from the optical receiver.
FIG. 2 is a block diagram of the optical receiver 110 in FIG. 1.
With reference to FIG. 2, the optical receiver 110 includes a photodiode 111 for converting the input optical signal Sin into an electrical signal, a transimpedance amplifier 112 for amplifying the electrical signal photoelectrically converted by the photodiode 111 at a predetermined gain, and a limiting amplifier 113 for limiting an output signal from the transimpedance amplifier 112 to a predetermined level and outputting the resulting signal Scout.
The output signal Scout from the limiting amplifier 113 has a cross point adjustable with a direct current (DC) voltage inputted thereto. This signal Sout is also composed of two differential signal components Sout1 and Sout2 with opposite phases such that it is processable even when at a low level.
The signal Scout photoelectrically converted and outputted by the optical receiver 110 shows distributions of ‘1’ and ‘0’ signal levels on the basis of the cross point thereof, which is subject to a variation resulting from the DC voltage to the limiting amplifier 113. This output signal Scout will hereinafter be described with reference to FIGS. 3a, 3b and 3c. 
FIGS. 3a, 3b and 3c are waveform diagrams showing level distributions of the output signal from the optical receiver 110 of FIG. 2.
Where the cross point of the output signal from the optical receiver 110 is 50% as shown in FIG. 3a, the output signal has a signal level ‘1’ distribution S1D and a signal level ‘0’ distribution S0D symmetrical to each other about the cross point. As a result, the signal level ‘1’ distribution S1D and signal level ‘0’ distribution S0D contain very small or consistent errors, so as to have substantially the same signal level ‘0’ error distribution S0ED and signal level ‘1’ error distribution S1ED at their one sides, respectively.
However, where the cross point of the output signal from the optical receiver 110 is not 50% as shown in FIGS. 3b and 3c, for example, where the output signal has a larger signal level ‘1’ error distribution S1ED as shown in FIG. 3b, the probability of the output signal being decided to be ‘0’ in level is higher although it must be decided to be originally ‘1’ in level. On the contrary, where the output signal has a larger signal level ‘0’ error distribution S0ED as shown in FIG. 3c, the probability of the output signal being decided to be ‘1’ in level is higher although it must be decided to be originally ‘0’ in level. That is, in the case where the signal cross point is not 50% as shown in FIGS. 3b and 3c, a larger amount of signal level decision errors take place in the process of data recovery by the clock/data recovery unit 120.
In particular, an optical signal is transmitted to the optical receiver via a plurality of optical amplifiers and a plurality of optical transmission lines. In this transmission process, the optical signal is compressed or spread due to dispersion and nonlinearity of the optical transmission lines. Further, noise is accumulated in level ‘1’ data of the optical signal due to spontaneous emission noise of the optical amplifiers. As a result, it is hard for the clock/data recovery unit to electrically make an accurate distinction between the level 1 and the level 0.
Therefore, in order to obtain the best data characteristics, or the lowest bit error rate, there is a need to, ahead of others, optimally adjust distributions of levels, or ‘1’ and ‘0’ levels, of the electrical signal from the optical receiver.